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3.

0 Energy Efficiency
in Industrial Utilities:
3.1 Boiler systems
A. K. SINHA
NATIONAL PRODUCTIVITY COUNCIL,
INDIA
Introduction to Boiler
 It is an enclosed Pressure
Vessel
 Heat generated by
Combustion of Fuel is
transferred to water to
become steam
 Process: Evaporation

 Steam volume increases to
1,600 times from water and
produces tremendous force
 Care is must to avoid explosion.
What is a boiler?
Boiler Specification
 Boiler Make & Year :XYZ & 2003

 MCR(Maximum Continuous Rating) :10TPH (F & A
100
o
C)

 Rated Working Pressure :10.54 kg/cm
2
(g)

 Type of Boiler : 3 Pass Fire tube

 Fuel Fired : Fuel Oil
 Heating surface : M
2

Boiler Regulation
IBR Steam Pipe means
any pipe through which
steam passes from a
boiler to a prime mover
or other user or both, if
pressure at which steam
passes through such
pipes exceeds 3.5
kg/cm
2
above
atmospheric pressure or
such pipe exceeds 254
mm in internal diameter
Steam Boilers means
any closed vessel
exceeding 22.75 liters
in capacity and which is
used expressively for
generating steam under
pressure

Boiler Systems
Flue gas system
Water treatment system
Feed water system
Steam System
Blow down system
Fuel supply system
Air Supply system

Boiler Types and Classifications

 Fire in tube or Hot gas through
tubes and boiler feed water in shell
side
 Fire Tubes submerged in
water
Application
 Used for small steam capacities
( upto 25T/hr and 17.5kg/cm
2

Water Tube Boiler
 Water flow through tubes
 Water Tubes surrounded
by hot gas
Application
 Used for Power Plants
 Steam capacities range
from 4.5- 120 t/hr
Characteristics
 High Capital Cost
 Used for high pressure
high capacity steam boiler
 Demands more controls
 Calls for very stringent
water quality
Performance Evaluation of
Boilers
What are the factors for poor efficiency?
Efficiency reduces with time, due to poor combustion,
heat transfer fouling and poor operation and
maintenance.Deterioration of fuel and water quality
also leads to poor performance of boiler.
How Efficiency testing helps to improve
performance?
Helps us to find out how far the boiler efficiency drifts
away from the best efficiency. Any observed abnormal
deviations could therefore be investigated to pinpoint
the problem area for necessary corrective action.

Boiler Efficiency
There are two methods of assessing boiler efficiency.

1) The Direct Method: Where the energy gain of the working
fluid (water and steam) is compared with the energy content of the
boiler fuel.
2) The Indirect Method: Where the efficiency is the difference
between the losses and the energy input.
Boiler Efficiency
Evaluation Method

 Stack temperatures greater than 200°C
indicates potential for recovery of waste
heat.
 It also indicate the scaling of heat
transfer/recovery equipment and hence
the urgency of taking an early shut down
for water / flue side cleaning.

 For an older shell
boiler, with a flue gas
exit temperature of
260
o
C, an economizer
could be used to reduce
it to 200
o
C, Increase in
overall thermal
efficiency would be in
the order of 3%.
 Condensing
economizer(N.Gas)
Flue gas reduction up
to 65
o
C
6
o
C raise in feed water temperature, by economiser/condensate recovery,
corresponds to a 1% saving in fuel consumption
3. Combustion Air
Preheating
 Combustion air preheating is an
alternative to feedwater heating.

 In order to improve thermal efficiency by
1%, the combustion air temperature must
be raised by 20
o
C.

4. Incomplete Combustion
(c c c c c + co co co co)
 Incomplete combustion can arise from a shortage of air or
surplus of fuel or poor distribution of fuel.
 In the case of oil and gas fired systems, CO or smoke
with normal or high excess air indicates burner system
problems.
Example: Poor mixing of fuel and air at the burner. Poor oil
fires can result from improper viscosity, worn tips,
carbonization on tips and deterioration of diffusers.
 With coal firing: Loss occurs as grit carry-over or carbon-
in-ash (2% loss).
Example :In chain grate stokers, large lumps will not burn
out completely, while small pieces and fines may block the
air passage, thus causing poor air distribution.
Increase in the fines in pulverized coal also increases
carbon loss.
5. Control excess air
for every 1% reduction in excess air ,0.6% rise in
efficiency.

The optimum excess air level varies with furnace design, type of burner,
fuel and process variables.. Install oxygen trim system

6. Reduction of Scaling
and Soot Losses
 In oil and coal-fired boilers, soot buildup on tubes acts as
an insulator against heat transfer. Any such deposits
should be removed on a regular basis. Elevated stack
temperatures may indicate excessive soot buildup. Also
same result will occur due to scaling on the water side.
 High exit gas temperatures at normal excess air indicate
poor heat transfer performance. This condition can result
from a gradual build-up of gas-side or waterside deposits.
Waterside deposits require a review of water treatment
procedures and tube cleaning to remove deposits.
 Stack temperature should be checked and recorded
regularly as an indicator of soot deposits. When the flue
gas temperature rises about 20
o
C above the temperature
for a newly cleaned boiler, it is time to remove the soot
deposits
7. Effect of Boiler Loading on
Efficiency

 As the load falls, so does the value of the mass
flow rate of the flue gases through the tubes. This
reduction in flow rate for the same heat transfer
area, reduced the exit flue gas temperatures by a
small extent, reducing the sensible heat loss.
 Below half load, most combustion appliances
need more excess air to burn the fuel completely
and increases the sensible heat loss.
 Operation of boiler below 25% should be avoided
 Optimum efficiency occurs at 65-85% of full loads
8. Boiler Replacement

if the existing boiler is :
Old and inefficient, not capable of firing cheaper
substitution fuel, over or under-sized for present
requirements, not designed for ideal loading
conditions replacement option should be
explored.
 Since boiler plants traditionally have a useful life
of well over 25 years, replacement must be
carefully studied.
Requirement for Evaluation of
Boiler Performance
 To assess the existing efficiency of the
boilers by both direct and direct methods.
 To compare the efficiency with design /
PG test values.
 Suggest ways to improve boiler efficiency
 To check general health of the equipment.
INSTRUMENTS REQUIRED
 Flue gas analysers
 Portable temperature indicator
 On-line instruments of boiler control room.
 Facilities of the chemistry lab or outside lab for coal /
ash / water analysis. (coal proximate or ultimate
analysis, un-burnt in bottom and fly ash, TDS, pH of
feed water / blow-down / condensate.
 Power analyser for power measurement of ID fan, FD
fan, ESP, crushers, BFP (boiler feed water pump) cool
handling plant/ash handling plant, etc.
AUDIT PROCEDURE
Activity - I
 Select the boiler for which the ENERGY audit to be
carried out.
 Collect specifications, design and performance
guarantee (PG) test data.
 Collect maintenance history, previous boiler inspection
details, running hours and any problems existing in the
system.
 Check availability and working condition of various on-
line and portable instruments required for measurements
during the trials.
 Observations should be made by running the boiler for a
minimum of six hours on fairly constant load, with
readings being logged at an interval of half an hour.
AUDIT PROCEDURE
Activity - II
 Take overall view of the boiler system comprising of fuel
handling plant, burners, ID fans, FD fans,
dampers/valves, steam piping, thermal insulation, etc.,
with a view to check general health of various
installations i.e., to observe noticeable leakages of
coal/oil/gas/air/steam/feedwater/ condensate and also to
check condition of thermal insulation, steam traps etc.

 During the trial run, take samples of coal/oil, bottom ash
and fly ash, feed water, boiler drum water, return
condensate and get them analysed (if possible sample of
each of the above should also be sent to a reputed and
reliable outside laboratory (to facilitate counter-checking
of in-house lab results).
AUDIT PROCEDURE
Activity - II
 The following parameters are to be tested :-
a) Raw Coal / Solid Fuel GCV, ash content, volatile
matter, fixed carbon, total moisture and HGI value.
b) Oil GCV, carbon, hydrogen, sulphur, moisture.
c) Gaseous Fuel GCV, carbon, hydrogen, sulphur.
d) Fly Ash & Bottom Ash quantity.
 Combustibles in fly and bottom ash and GCV.
 Carry out power measurements of FD fan, ID fan, boiler
feed water pump, crushers, ash slurry pumps, etc.
AUDIT PROCEDURE
Activity - III
 Collect sample of flue gas and measure temperature, CO,
CO2 and O2, at different locations, i.e., before economiser,
after economiser, before air preheater and after air preheater,
before ESP and after ESP. Using on–line analysis ,portable
analysers, orsat apparatus.
 During trials, also take note of key unit parameters like boiler
steam flow, steam pressure, steam temperature, etc.
 Measure furnace draft at combustion chamber and inlet to ID
fan.
 Fuel flow rate and fuel consumed during the trial duration is to
be measured.
 Various other parameters observed should be logged on the
log sheet provided in Annexure-I.
AUDIT PROCEDURE
Activity - IV
 Calculation may be performed as per formula given in
auditors tools.
1) Efficiency of the boiler by direct method and
evaporation ratio.
2) Blow-down percentage calculation.
 List out scope for improvement.
 List out recommendations and actions to be taken for
improvement.
 Cost benefit analysis with savings potential for initiating
improvement measures.
AUDIT GUIDELINES FOR BOILERS
Storage and preparation of fuel :
For Solid Fuel
 Stack coal in neat heaps not exceeding 1.5 metres in height
and limit the individual heaps to 200 MT.
 Stack coal on hard ground and ensure a tightly packed
heap to subdue ventilation which can be lead to
spontaneous combustion.
 Size coal properly as per requirements.
 Wet coal carefully according to the content of fines in the
coal.
For Liquid Fuel
 Pre-heat the oil as per the manufacturers recommendation.
 Water draining periodically from storage tanks.
 Oil lines heat tracing.
AUDIT GUIDELINES FOR BOILERS
Combustion
 Periodic sampling of exit flues gases and
corresponding adjustment of air supply by dampers
are important to establish optimum level of CO2/O2
percentage (CO2 at 12 -14% and O2 at 3 - 6%) &
excess air
 Check for black smoke and adjust damper to get light
brown smoke.
 10 % additional Excess Air Reduces furnace efficiency
by 1.45 %
AUDIT GUIDELINES FOR BOILERS
Waste Heat Recovery
 Pre-heat combustion air by air pre-heater. For every
22
0
C rise in air temperature, there is 1% decrease in
fuel consumption.

BOILERS-DO’s& DON’Ts (contd.)
15. Keep fire-fighting arrangements in readiness
always. Mock rehearsals should be carried out
once a month.
16 . All log-sheets must be properly.
17. Check FD/ID inter locks wherever available
18. CO2 and O2 recorder must be checked and
calibrated.

Furnace should be designed so that in a
given time, as much of material as
possible can be heated to an uniform
temperature as possible with the least
possible fuel and labour.

Furnace Energy Supply

 The products of flue gases directly contact the
stock, so type of fuel chosen is of importance.
For example, some materials will not tolerate
sulphur in the fuel. Also use of solid fuels will
generate particulate matter, which will interfere
the stock place inside the furnace.
Hence, majority of the furnaces use liquid fuel,
gaseous fuel or electricity as energy input.

 Furnace oil is the major fuel used in reheating
and heat treatment furnaces
 LDO is used in furnaces where presence of
sulphur is undesirable.(No problom with sulphur
)
 Furnaces operate with efficiencies as low as 7%
as against upto 90% achievable in other
combustion equipment such as boiler.
 This is because of the high temperature at which
the furnaces have to operate to meet the
required demand. For example, a furnace
heating the stock to 1200
o
C will have its exhaust
gases leaving atleast at 1200
o
C resulting in a
huge heat loss through the stack.
Heat Transfer in Furnaces

Figure 4.3 : Heat Transfer in furnace
•Radiation from the
flame,hot combustion
products and the
furnace walls and
roof;
•Convection due to
the movement of hot
gases over the stock
surface.
Performance Evaluation of a Typical
Furnace
Figure 4.10 Heat losses in industrial heating Furnaces

Stored Heat Loss:
Wall Loss:
Furnace Efficiency Calculation
Example: by direct Method
An oil-fired reheating furnace has an operating
temperature of around 1340
o
C. Average fuel
consumption is 400 litres/hour. The flue gas exit
temperature after air preheater is 750
o
C. Air is
preheated from ambient temperature of 40
o
C to 190
o
C
through an air pre-heater. The furnace has 460 mm thick
wall (x) on the billet extraction outlet side, which is 1 m
high (D) and 1 m wide. The other data are as given
below. Find out the efficiency of the furnace by direct
method.
Trial Data
Flue gas temperature after air preheater =750
o
C
Ambient temperature =40
o
C
Preheated air temperature =190
o
C

The amount of heat lost in the flue gases
depends upon amount of excess air. In the
case of a furnace carrying away flue gases at
900
o
C, % heat lost is shown in table :
Table Heat Loss in Flue Gas Based on Excess Air Level

Temperature for Different Furnaces
4) Reducing Heat Loss from Furnace
Openings
The heat loss from an opening can be calculated using the formula:
Q=4.88 x T
4
x a x A x H … k.Cal/hr
100
T: absolute temperature (K),
a: factor for total radiation
A: area of opening,
H: time (Hr)
Heat loss through openings consists of direct radiation and
combustion gas that leaks through openings.

Keeping the doors unnecessarily open leads to wastage of fuel
Inspection doors should not kept open during operation
Broken and damaged doors should be repaired
5)Maintaining correct amount of
furnace draught
Negative pressures : air infiltration- affecting air-fuel ratio control,
problems of cold metal and non-uniform metal temperatures,
Positive Pressure: Ex-filtration -Problems of leaping out of flames,
overheating of refractories,burning out of ducts etc.
6) Optimum capacity utilization

There is a particular loading at which the furnace will operate at
maximum thermal efficiency.
Best method of loading is generally obtained by trial-noting the weight
of material put in at each charge, the time it takes to reach temperature
and the amount of fuel used.
Mismatching of furnace dimension with respect to charge and production
schedule.
Coordination between the furnace operator, production and planning
personnel is needed.
7) Waste heat recovery from the flue
gases

• Charge (stock) preheating,

•Preheating of combustion air,

•Utilizing waste heat for other process

8. Minimizing Wall Losses
About 30% of the fuel input to the furnace generally goes to
make up for heat losses in intermittent or continuous furnaces.
The appropriate choice of refractory and insulation materials is
needed for high fuel savings in industrial furnaces.

The extent of wall losses depend on:
Emissivity of wall
Thermal conductivity of refractories
Wall thickness
Whether furnace is operated continuously or
intermittently
Radiation Heat Loss from Surface of
Furnace
The quantity (Q) of heat release from a reheating furnace is
calculated with the following formula: